(1) Field of the Invention
The present invention relates to a method for spraying a pulverulent material into a carrier gas having a total flow rate, said method comprising                a flow of said pressurized carrier gas,        an acceleration of said pressurized carrier gas to a sonic velocity,        an expansion of said pressurized carrier gas with formation of a negative pressure zone having a value lower than said carrier gas flow pressure and an entrainment of an amount of said pulverulent material by said expanded carrier gas, and        a spray of said pulverulent material entrained by said carrier gas.        
(2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98.
Such a method is known for example from document U.S. Pat. No. 6,402,050, which describes apparatus for the dynamic spraying of pulverulent materials by gases in the field of the production of coatings, for example corrosion-resistant or reflecting coatings for machined surfaces.
This document describes the use of a sonic throat with a particular ratio of the cross sections between the sonic throat and the pulverulent material feed, in order to maintain a pressure lower than atmospheric pressure for transporting the powder by an air stream at atmospheric pressure. This document does not disclose that the sonic nozzle serves to obtain a constant flow rate of pulverulent material.
However, in the field of the repair of the refractory furnace walls by flame spraying, gunning, ceramic welding or reactive spraying, the reproducibility of a method for spraying pulverulent material and all the adjustments pertaining thereto, like those of the amount of pulverulent material, the spray velocity, the impact force, etc., are directly and harmfully affected by a non-reproducible variable carrier gas flow rate.
Obviously, devices are known for comprising a flowmeter which, via a controller, controls a valve to obtain a constant gas flow rate, but such systems are complicated to apply and demand elements of which the purchase price and operation are directly related to the accuracy. Accordingly, these systems are relatively inapplicable, not to mention the fact that the final accuracy (probably due to the sequence of elements) is often insufficient.
Furthermore, certain known methods in the field of repair by spraying pulverulent material comprise an adjustment of the amount of pulverulent entrained by means of an endless screw or a discharge revolving table, but the use of such entrainment devices requires the use of electric motors, which is incompatible with the use of a carrier and reactive gas (for example oxygen) with at least one element of said pulverulent material.
To ensure the safe use of these electric motors, an inert gas would have to be used, for example nitrogen, but this is incompatible with the inventive method, because the carrier gas must be reactive with an element of the pulverulent material and in every case requires an additional nitrogen feed, making the method less flexible.
It is therefore an object of the invention to overcome these drawbacks by providing a method in which the flow rate of pulverulent material is adjustable and reproducible without affecting the carrier gas flow rate.
For this purpose, the inventive method is characterized in that it further comprises an adjustment of said lower pressure, which exists in the negative pressure zone by the bypassing or not, before the expansion, of an adjustable amount of said carrier gas having been accelerated to reintroduce said adjustable amount into the aforesaid negative pressure zone without changing said flow rate, in particular in its totality.
The amount of instantaneous pulverulent material entrained should advantageously be optimized with regard to the excellence of the coating, but also from the standpoint of the cost of consumption thereof. Upstream of the spray pipe or nozzle, it is therefore important to be able to mix the pulverulent material intimately with an adjustable amount of carrier and reactive gas. Accordingly, the value of the latter parameter is also dictated by necessity.
The inventive method as described above has the desired flexibility with regard to a conventional method using a venturi effect. This is because the spray method according to the invention, by comprising a step of adjustment of said negative pressure by the bypassing or not, before the expansion, of an adjustable quantity of carrier gas having been accelerated, makes it possible, while making no change to the carrier gas outlet flow rate, to change the value of the lower pressure in the negative pressure zone, thereby serving to adjust the amount of pulverulent material entrained.
If the amount of carrier and reactive gas withdrawn and reintroduced is high, the value of the pressure in the negative pressure zone is closer to the compression pressure and the amount of entrained pulverulent material is lower. On the contrary, if the amount of carrier and reactive gas withdrawn and reintroduced is low, the value of the pressure in the negative pressure zone is sharply decreased in comparison with the aforesaid compression pressure value and a large amount of pulverulent material close to its maximum value will also be entrained. If the amount of bypassed carrier gas is zero, the value of the negative pressure is a maximum and has the furthest value from the compression pressure that the method can reach, and the maximum amount of pulverulent material is entrained. Accordingly, the amount of carrier and reactive gas bypassed (that is withdrawn and reintroduced) serves to very advantageously adjust the amount of pulverulent material entrained.
The invention has therefore served to overcome at least part of the drawbacks of the prior art by allowing the adjustment of the amount of entrained pulverulent material to a reproducible value, while ensuring a constant carrier gas flow rate, thereby guaranteeing a constant ejection speed. In fact, the final result, the reproducibility and the quality of the spraying, depend directly on this flow rate of pulverulent material entrained by said carrier gas.
An optimal carrier gas flow rate ensures optimal transport of the material to be sprayed, and since the spraying is carried out via a spray pipe or nozzle, having a clearly defined spray cross section, the spray velocity for a given carrier gas temperature is therefore conditioned by the flow rate of this carrier gas.
Thanks to the acceleration to sonic velocity, for example obtained by creating a shock wave in a venturi, the sonic barrier establishes a fixed flow rate which is not influenced by the pressure drop variations in the downstream circuit. Accordingly, the carrier gas flow rate has become constant and the spray velocity condition by this constant flow rate is optimal. The optimal ejection speed thus obtained in the carrier gas considerably increases the reliability and reproducibility of the inventive method for spraying pulverulent material.
In the field of the repair of refractory walls of furnaces, glass treatment installation, coke ovens, etc., the inventive method can be applied advantageously in a reactive spraying repair method which consists in spraying a pulverulent material (comprising for example a refractory filler and metal powder), finely atomized, by means of a carrier gas stream on a target zone.
In fact, when a refractory wall has superficial or deep damage, the user must repair it as quickly as possible to avoid aggravating the damage, considering the intense operating conditions.
During the reactive spraying repair operation, the quality of the coating obtained on the generally refractory wall depends on several parameters, including in particular the temperature of the support and the spray velocity.
In this type of method, the carrier gas may also advantageously be a gas that is reactive with at least one of the elements of the pulverulent material and, in contact with the hot wall, the mixture reacts spontaneously and a series of chemical reactions leads to the formation of a homogenous, adhesive refractory material whose properties are compatible with those of the support treated.
The spray velocity is a predominant factor. This is because if it is too low, there is a risk of flashback. If it is too high, the amount of material may not react (because it does not participate in the exothermic reaction) and may rebound excessively on the wall, to the detriment of the quality of the magma under formation caused by the reactive spraying.
It is therefore an object of the inventive method to obtain an optimal weld quality by procuring a quality of spraying and impact of said pulverulent material on the surface to be repaired that is constant over time. The inventive method is suitable for obtaining a carrier and reactive gas flow rate that depends directly on the inlet pressure that is independent of any change in pressure resulting from the downstream circuit.
The grains making up the sprayed pulverulent material are activated within optimized velocity thanks to the carrier gas which transports the pulverulent material pneumatically and the amount is adjustable.
In this type of reactive spraying repair application, the carrier gas is also a reactive gas which serves not only as a transport fluid but also participates actively in the exothermic physicochemical reaction. The final quality of the sprayed project essentially depends on the following factors:                the total enthalpy produced during the exothermic reaction depends on the amount of carrier and reactive gas used, and also the temperature, chemical composition or formulation of the pulverulent material,        the amount of powder sprayed, that is the mass flow rate of pulverulent material,        the optimal flow rate of carrier and reactive gas used to obtain the optimal ejection speed of the reactants for a given application.        
Since the carrier gas flow rate, according to the invention, advantageously has a constant value at the outlet, free of any variation due to imperfections, the inventive method provides an optimal spraying velocity for a given application.
Advantageously, the inventive method further comprises a compression of said reactive carrier gas having been accelerated previously to the expansion, thereby serving to improve the entrainment of the aforesaid pulverulent material.
Other embodiments of the inventive method are mentioned in the appended claims.